1. Field of the Invention
The present invention relates to a substrate for magnetic recording medium and to a method for manufacturing the substrate for magnetic recording medium.
2. Description of the Related Art
The recording density (surface density) of magnetic recording has increased extremely rapidly and continuously at yearly rates of 50 to 200% for these past 10 years. At the mass production level, products with 70 Gbits/inch2 are shipped, while surface recording densities several times higher, namely 160 Gbits/inch2, have been reported at the laboratory level. Surface recording densities at the mass production level correspond to 80 Gbytes per one platter of a 3.5″ HDD (3.5 inch HDD), and correspond to 40 Gbytes per single platter of a 2.5″ HDD. At these recording volumes, installation of single platter recording media gives a sufficient volume for use in an ordinary desk top personal computer (equipped with a 3.5″ HDD) or a laptop personal computer (equipped with a 2.5″ HDD).
It is expected that recording densities will also continue to improve in the future. However, the conventional horizontal magnetic recording methods are approaching their thermal fluctuation recording limit, and it seems that they will be replaced by perpendicular magnetic recording when recording densities of 100 Gbit/inch2 to 200 Gbit/inch2 are reached. At the present time it is not certain what the recording limit of perpendicular magnetic recording will be, but it is believed that 1000 Gbit/inch2 (1 Tbit/inch2) is achievable. If these types of high recording densities are achieved, it will be possible to obtain a recording volume of 600 to 700 Gbytes per single platter of a 2.5″ HDD.
In order to realize such a high recording density, the flying height of the magnetic recording head must be reduced from the conventional 30 nm to 10 nm or less, and smoothing of the substrate surface is essential. However, it has recently been found that problems such as the head adhering to the substrate or a loss of head flying stability occur when micro level surface roughness is too small. That is, a substrate having extremely low waviness and micro waviness and having a roughness of about 0.3 to 2.0 nm is ideal. (Regarding unevenness of a substrate to be used for a magnetic recording medium having a magnetic film, waviness stands for an observable range of 5 to 100 mm, micro waviness stands for an observable range of 80 μm to 5 mm, and roughness stands for an observable range of up to 80 μm.)
At present, glass substrates are textured by tape polishing, but the desired roughness is not achievable and it is not possible to lower the roughness to the desired head height.
The methods for surface-roughening silicon substrates include a dry etching method with chlorine (Japanese Patent Application Unexamined Publication No. 7-263406/1995) and a method of treatment with alkali hydroxide (Japanese Patent Application Unexamined Publication No. 53-7144/1978). However, although it is possible to control the degree of surface roughening with these methods, it is not possible to finish the entire substrate with a uniform roughness. That is, because the substrate is etched while the substrate rests motionless in a non-uniform atmosphere during etching, selective etching occurs and defects form due to the effect of process strains residual on the substrate. The reality at present is that it is not possible to obtain a uniform roughness with such simple acid or alkali etching.